Page 31 - Boiler Operator’s Handbook

16 Boiler Operator’s Handbook

relationship between pressure drop and flow back in
the seventeenth century and, since it’s a natural law of
physics, we’ll continue to use it. In order for air to flow
from one spot to another, the pressure at spot one has
to be higher than the pressure at spot two. It’s the same
as water flowing downhill. The higher the pressure dif-
ferential the faster a fluid will flow. If you think about
the small changes in atmospheric pressure causing the
wind, you know it doesn’t take a lot of difference in
pressure to really get that air moving. Bernoulli discov-
ered the total pressure in the air doesn’t change except
for friction and that total pressure can be described as
the sum of static pressure and velocity pressure.
The measurement of static pressure, velocity pres-
sure, and total pressure is described using Figure 1-5.
The static pressure is the pressure in the fluid measured Figure 1-5. Static, velocity, total measurements
in a way that isn’t affected by the flow. Note that the
connection to the gage is perpendicular to the flow. The PRESSURE DROP AND FLOW
gage measuring total pressure is pointed into the flow
stream so the static pressure and the velocity pressure There’s another thing about flow that’s important
are measured on the gage. What really happens at that to understand; a change in pressure drop is proportional
nozzle pointed into the stream is the moving liquid to the square of a change in flow. You’ll recall from high
slams into the connection converting the velocity to school math that you multiply a number by itself to
additional static pressure sensed by the gage. There is square it. In my boiler operator training classes I use the
no flow of fluid up the connecting tubing to the gauge. diagram (Figure 1-6) of a sprinkler on the lawn hooked
The measurement of velocity pressure requires a special up to a hose after a tee with a pressure gage that reads
gage that measures the difference between static pres- 10 psig and ask the operators what the gage should
sure and total pressure. With that measurement we can read when the flow is twice as much as shown. Almost
determine the velocity of the fluid independent of the invariably I get the answer of “20 psig” but I also get a
static pressure. A velocity reading in a pipe upstream of bunch of wild guesses and it was two years before I got
a pump, where the pressure is lower, would be the same the right answer after hundreds of classes. If I want two
as in a pipe downstream of the pump (provided the pipe times the flow the pressure drop has to increase by four
size is the same). times (two squared is four) and the gage has to read 40
If you’ve never played in the creek before, go give psig.
it a try to see how this works. Notice the level of water I also demonstrate to those classes that fractions
leaving a still pool and flowing over and between some can be used to compare different situations. Imagine a
rocks. Put a large rock in one of the gaps and you’ll re-
duce the water flow through that gap but that water has
to go somewhere. The level in the pool will go up, prob-
ably so little that you won’t notice it because the water
flow you blocked is shared by all the other gaps and the
only way more water can flow is to have more cross-
section to flow through. I think I learned more about
hydraulics (the study of fluid flow) from playing in the
creek in my back yard than I ever learned in school. You
could gain some real insight into fluid flow by spending
some time observing a creek. That’s a creek, now, not a
large deep river. All the education is acquired by seeing
how the water flows over and through the rocks and
relating what you see to the concepts of static, velocity,
and total pressure. 1-6. A lawn sprinkler example